1. Field of the Invention
The present invention relates to a structure having a photonic crystal and a surface-emitting laser using the same.
2. Description of the Related Art
In recent years, researches concerning a photonic crystal have been actively performed. The photonic crystal is the structure in which, like a band gap present in an electronic state in a crystal, a wavelength band (a photonic band gap) for controlling light waveguide is generated by giving periodic structure to a refractive index.
It is known that incident light is diffracted in an in-plane direction of the photonic crystal and diffracted light of the incident light is combined with a radiation mode and transmitted or reflected.
Guided Resonance (GR) is a phenomenon in which light of a waveguide mode guided in the in-plane direction of the photonic crystal having the refractive index periodic structure in the in-plane direction is combined with the radiation mode to be radiated to the outside of the photonic crystal.
The photonic crystal can be used as a mirror by using the GR. When light made incident in a direction perpendicular to the in-plane of the photonic crystal is combined with a mode located above a light line, the light is converted into a mode for waveguide in the photonic crystal. Thereafter, when the light is combined with the radiation mode, the light is radiated to the outside of the photonic crystal again. An unusual reflection phenomenon, for example, reflection with efficiency of 100% is caused by interference between light that is directly reflected without being combined with the waveguide mode and light that is once combined with the waveguide mode and radiated to the outside again.
In general, the waveguide mode means a mode in which a condition that light does not leak from a photonic crystal layer is satisfied. The radiation mode means a mode in which light leaks from the photonic crystal layer.
The light line indicates a dispersion relation of light that propagates in a medium adjacent to a waveguide layer (in this case, the photonic crystal layer). The light line can be indicated by a straight line decided by w=ck/n (w: angular frequency, c: light velocity, n: refractive index, k: wave number). In general, an area higher in frequency than the light line is an area in which light tends to leak from the photonic crystal.
As described above, the mirror using the GR has an operation principle different from that of a mirror using the photonic band gap. Non-Patent Document Appl. Phys. Lett. 87, 091102 (2005) describes an examination concerning GR performed by using a photonic crystal. When a wavelength of light made vertically incident on the photonic crystal illustrated in FIG. 2A of the document is adjusted to a wavelength at which reflectance is extremely increased by the GR, the incident light is reflected at high reflectance.
In the Non-Patent Document, a simulation is performed with a radius r of pillars circular in section set to 0.4a (“a” is a period of a grating), height h set to 0.9a, a refractive index nphc of a material forming the pillars set to 4.47, and a refractive index of an area adjacent to the pillars set to 1.0. As a result, GR illustrated in FIG. 2B occurs.
When a mirror applied to a laser and the like is manufactured using the GR in the photonic crystal, a refractive index difference between a photonic crystal layer and a cladding layer adjacent thereto may have to be extremely small. For example, in a surface-emitting laser having a wavelength of 670 nm, it is assumed that the photonic crystal layer is formed of Al0.5Ga0.5As and the cladding layer adjacent thereto is formed of Al0.93Ga0.07As or the like. In this case, refractive indexes of Al0.5Ga0.5As and Al0.93Ga0.07As with respect to light having a wavelength of 670 nm are 3.446 and 3.130, respectively. Thus, a relative refractive index difference Δn(=(nphc−nclad)/nphc)) between both the light is about 0.09 (about 9%).
In such a case, it is anticipated that, if the structure described in the Non-Patent Document is directly applied, the GR hardly occurs and it is difficult to use the photonic crystal as a mirror.
Therefore, the inventors performed a simulation using the structure illustrated in FIG. 3A in the same manner and checked whether the GR could be present. The inventor confirmed that the GR could occur when a relative refractive index difference Δn(=(nphc−nclad)/nphc) between a photonic crystal layer including pillars and a substrate functioning as a cladding layer adjacent thereto is not less than 0.35 (35%) but the GR hardly occurs when the relative refractive index difference does not exceed 0.13 (13%). An actual simulation result is illustrated in FIG. 3B.